This invention relates to control circuits for injection lasers and, more particularly, to a control circuit for automatically adjusting the bias of an injection laser.
Information can be transmitted on the output light of an injection laser by modulating the current through the laser in accordance with the transmitting signal, the latter being either an ac sinusoidal signal or a pulse train. In order, however, for the output light to be linearly related to the magnitude of the input modulating signal, the laser current must remain above the threshold level for all modulating input signal magnitudes. If the modulating signal causes the laser current to fall below the threshold level, the output light is "clipped" and when the modulated light is detected by a receiver, the recovered signal is a distorted replica of the modulating signal. Furthermore, the threshold level of an injection laser drifts as the laser ages and is a function of the ambient temperature. A signal which modulates the laser current without distortion at one point in time and at one temperature may drive the laser current below the threshold level due to a threshold drift. Accordingly, the laser must be biased with a dc current such that the laser current, when modulated, remains above the threshold level.
Prior art circuits have automatically adjusted the bias of an injection laser by optically detecting the output light of the laser and deriving a feedback signal to control the bias of the laser by comparing the electrical signal at the output of the optical detector with the input modulating signal. The use of an optical detector, however, requires that one face of the laser be accessible for feedback purposes or requires apparatus to divert a portion of the useful light output of the laser. Moreover, an optical link in the feedback network increases the complexity and cost of the control apparatus.
An object of the present invention is to automatically stabilize the bias current of an injection laser such that the laser operates above the threshold level.